الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 93 |  |  |
| | | from | 93 | | |
Abstract
Electrically conductive composites consisted of conducting fillers and the insulating polymer matrix. These composites could serve in preparation of piezoresistive composites(i.e. quantum tunnelling composites, QTC) due to their flexibility, light weight, easy processing, low cost, greater resistance changes, and ease of spreading over arbitrary curved surfaces. QTC have attracted tremendous attention due to their potential applications in advanced stress and strain sensors. Recently, various types of conducting materials and many soft polymers have been utilized in the manufacture of QTCs. characterization of such composites involves several physical parameters. Therefore, a low cost technique was designed and manufactures in order to measure most of electro-mechanical properties of the produced composites with good accuracy and repeatability. QTCs were prepared by mixing poly-dimethyl siloxane (PDMS) with different concentrations of graphite flakes (1:0.75, 1:1,
1:1.5, 1:1.75 and 1:2). Conductivity of the composites containing higher concentration of graphite was found to be independent on rate of pressing during the test. A relation between the capacitance of the sample and the applied pressure has been derived theoretically and then verified practically. It was also found that the capacitive behaviour of the sample interrupted the flow of current at the instant of removing the applied pressure. At the application of alternating voltage it is found that a piece of the QTC behaves as band pass filter. The impedance and the filter peak frequency of a QTC were measured. It is found that the peak frequency decreased as the ratio of graphite in the composition increased. Scanning electron microscopic images show that; graphite flacks has sharp tips and sharp edges that could act as spicks which concentrate charges to produce intense localized fields. Also it is observed that the presence of graphite powder in PDMS prevents the good coating of carbon nano-fibres(CNF) with polymer. PDMS mixed with copper nano-particles show very good wetability of CNF with the polymer. The suggested setup has several advantages such as simplicity, high accuracy and providing lots of technical data that required for development and confirmation of models for the quantum tunnelling process.